Hi Puterwade,
If I understand correctly, your 27" iMac shows the Apple logo and plays the startup chime when you press the power button, and then the screen immediately goes black but the Mac remains powered on?
If that's correct, you might want to try starting up from macOS Recovery to see if it's a software or firmware problem. Fully power off your iMac, connect a wired keyboard, then power on your iMac while holding down Command-R. Release the keys when the Apple logo and a progress bar appear.
If you're using a Windows keyboard, try holding down Windows-R or Alt-R as you power on the iMac.
Please let me know whether the issue still persists when trying to access macOS Recovery, or if you can boot into Recovery without issues.